Observation of aerodynamic instability in the flow of a particle stream in a dilute gas

Abstract

Forming macroscopic solid bodies in circumstellar discs requires local dust concentration levels significantly higher than the mean. Interactions of the dust particles with the gas must serve to augment local particle densities, and facilitate growth past barriers in the metre size range. Amongst a number of mechanisms that can amplify the local density of solids, aerodynamic streaming instability (SI) is one of the most promising. This work tests the physical assumptions of models that lead to SI in protoplanetary disks (PPDs). We conduct laboratory experiments in which we track the three-dimensional motion of spherical solid particles fluidized in a low-pressure, laminar, incompressible, gas stream. The particle sizes span the Stokes-Epstein drag regime transition and the overall dust-to-gas mass density ratio is close to unity. Lambrechts et al. (2016) established the similarity of the laboratory flow to a simplified PPD model flow. We obtain experimental results suggesting an instability due to particle-gas interaction: i) there exist variations in particle concentration in the direction of the mean drag forces; ii) the particles have a tendency to 'catch up' to one another when they are in proximity; iii) particle clumping occurs on very small scales, which implies local enhancements above the background dust-to-gas mass density ratio by factors of several tens; v) the presence of these density enhancements occurs for a mean dust-to-gas mass density ratio approaching or greater than 1; v) we find evidence for collective particle drag reduction when the local particle number density becomes high and when the background gas pressure is high so that the drag is in the continuum regime. The experiments presented here are precedent-setting for observing SI under controlled conditions and may lead to a deeper understanding of how it operates in nature.